The Sun can lash the Earth with powerful winds that can disrupt communications, aviation and power lines even when it is in the quiet phase of its 11-year solar cycle, say US scientists.

Observers have traditionally used the number of sunspots on the surface of the Sun to measure its activity. The number of sunspots reaches a peak at what is called the solar maximum and then declines to reach a minimum during a cycle.

"The Sun continues to surprise us," says Dr Sarah Gibson of the Center's High Altitude Observatory and lead author of the study. "The solar wind can hit Earth like a fire hose even when there are virtually no sunspots."

Scientists previously thought the streams of energy largely disappeared as the solar cycle approached the minimum.

Ever present

Gibson and the team, which also included scientists from NOAA and NASA, compared measurements from the current solar minimum interval, taken in 2008, with measurements of the last solar minimum in 1996.

Although the current solar minimum has fewer sunspots than any minimum in 75 years, the Sun's effect on Earth's outer radiation belt was more than three times greater last year than in 1996.

The research, published in the latest issue of the Journal of Geophysical Research, found that the prevalence of high-speed streams during the solar minimum in 2008 appeared to be related to the current structure of the Sun.

As the number of sunspots fell over the past few years, large holes lingered in the surface of the Sun near its equator. The high-speed streams that blow out of those holes engulfed Earth during 55% of the study period in 2008, compared to 31% of the study period in 1996.

A single stream of charged particles can last for as long as 7 to 10 days, the study says.

"The new observations from last year are changing our understanding of how solar quiet intervals affect the Earth and how and why this might change from cycle to cycle," says co-author Dr Janet Kozyra of the University of Michigan.

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